The present invention relates to a cut-off amplifier and is applicable to the regulation of the supply voltage of a load, such as a motor, and a more particularly a motor of autosynchoronous type. For example, this motor can be of the type used in robotics. In this special field, the motors at times absorb electrical energy in order to supply mechanical energy and at times absorb mechanical energy for restoring the electrical energy to the source which supplies them.
Cut-off amplifiers are known, which in particular make it possible to supply direct current motors. These cut-off amplifiers are of two types and cut-off can take place either from two d.c. voltages, one positive and one negative, or from the a.c. mains.
A cut-off amplifier operating from the d.c. voltages, one positive and the other negative, is shown in FIG. 1. This cut-off amplifier comprises two d.c. power sources E1, E2, which are connected to a reference earth R. Source E1, which supplies a positive voltage with respect to earth, is connected to the load (e.g. a motor M), via a reverse-connected diode D1 and a choke L. The negative voltage E2 is connected to motor M, via a reverse-connected diode D2 and a choke L. Transistors T1, T2, e.g. of the bipolar type, are respectively connected between the positive terminal of source E1 and a terminal of choke L, and between the negative terminal of source E2 and the terminal of choke L. These switching transistors receive switching signals at their bases B1, B2. These transistors are equivalent to switches and are controlled, by not shown means, as a function of the cut-off of d.c. voltages making it possible to obtain the desired regulation.
When the motor M absorbs electrical energy and restores mechanical energy, a current I passes through choke L in the direction indicated by arrow 1. In this case and in a first phase, transistor T1 is made conductive, whilst transistor T2 is off. The currents in the different branches of the circuit are represented by arrows 2 and 3 and source E1 supplies energy. In this case and in the second phase, transistor T1 is off and transistor T2 conductive, but no current flows therein. The current is represented by arrows 4 and 5 and the source E2 absorbs energy.
Conversely, when the motor absorbs mechanical energy and restores electrical energy, the current in choke L flows in the direction indicated by arrow 6. In this case and in a first phase, transistor T2 is made conductive and transistor T1 is off. The currents in the different branches of the circuit are represented by arrows 9 and 10 and the negative voltage source E2 supplies energy. In this case and in a second phase, transistor T2 is off and transistor T1 is made conductive, but no current flows therein. The current is represented by arrows 7 and 8 and the positive voltage source E1 absorbs energy.
Thus, in both cases, one of the sources supplies energy, whilst the other receives it.
The main disadvantage of the cut-off amplifier according to FIG. 1 is that only rarely are storage batteries available, these being heavy, expensive, cumbersome, dirty and of indifferent reliability, whilst two d.c. mains, one positive and the other negative, are never available.
A cut-off supply operating from an a.c. mains is obtained by replacing sources E1 and E2 by two connections of the type shown in FIG. 2. For example, each connection comprises an input transformer T, whose secondary has a centre connected to a reference earth R. The ends of this secondary are connected to two diodes D3, D4, making it possible to obtain at output S, a rectified voltage. A filter capacitor C connects said output to the reference earth. If output S of the cut-off supply obtained is connected, as in the preceding example, to a motor M, no problem is caused when the motor receives electrical energy and supplies mechanical energy. However, when the motor receives mechanical energy and restores the electrical energy to the mains, the voltage at the terminals of capacitor C increases rapidly. When this voltage becomes excessive, there is a destructive breakdown of the capacitor, and possibly of transistors T1, T2 and diodes, D3 and D4. It is therefore necessary to provide a system discharging the capacitors, e.g. across a resistor, when the voltage at its terminals exceeds a certain value. This generally relatively complicated system obviously requires the removal of the thermal energy by means of radiators or fans. This is a serious disadvantage, because in general, low overall dimensions of cutoff amplifiers are sought, and generally any heat removal system has large dimensions.
The present invention aims at obviating the disadvantages of cut-off amplifiers operating from the a.c. mains and more particularly aims at providing an a.c. voltage cut-off amplifier by means of which it is possible to restore energy to the mains, which is not possible in the case of cut-off amplifiers operating from d.c. voltages obtained by rectification of the mains.